1. Field of the Invention
The present invention relates to a process for the production of alcohols by hydroformylating olefins with synthesis gas in the presence of an organic phase containing a cobalt catalyst and subsequent hydrogenation of the aldehydes thus obtained. The cobalt catalyst is formed by reacting an aqueous cobalt salt solution in the presence of an organic solvent, which is only slightly miscible with water or not at all, with synthesis gas. The organic phase containing the cobalt catalyst is obtained by extraction of the cobalt catalyst formed from the aqueous phase by means of an organic extractant which is only slightly miscible with water or not at all.
In addition, the invention relates to the use of the alcohols produced for the production of carboxylic acid esters as plasticizers for plastics.
2. Discussion of the Background
The hydroformylation of olefins with carbon monoxide and hydrogen to give aldehydes having one more carbon atom than the olefin in the presence of transition metal catalysts such as, for example, cobalt and rhodium compounds, is known as the oxo synthesis. In general, a high proportion of straight-chain aldehydes, which are the intermediates in the production of the economically important plasticizer alcohols for plastics and detergent alcohols, is desired in the hydroformylation of olefins to give aldehydes.
While linear and terminal olefins (so-called .alpha.-olefins) can very readily be hydroformylated with phosphine-modified rhodium or cobalt catalysts (J. Falbe, Editor, "New Synthesis with Carbon Monoxide", Springer-Verlag, Berlin 1980, pages 55 et seq.,) unmodified cobalt and rhodium catalysts are preferentially employed for low-reactivity olefins, internal olefins, and internal and branched olefins.
In the presence of modified catalysts, internal and branched olefins are hydroformylated very slowly or only partially. This precludes the potential use of modified catalysts for the economical hydroformylation of internal and branched olefins.
The hydroformylation of polymeric and isomeric olefin mixtures containing terminal and internal as well as internal and branched olefins is advantageously carried out with unmodified cobalt catalysts. As compared with rhodium catalysts, higher yields of the valuable straight chain aldehydes are obtained with cobalt catalysts, starting from the same initial olefin.
Examples of typical polymeric and isomeric olefin mixtures, which are preferably converted by cobalt-catalyzed hydroformylation to give the corresponding oxo aldehydes, are the dimers, trimers and tetramers of propene, n-butenes (1- and 2-butene), and isobutene.
According to the known processes, cobalt-catalyzed hydroformylation is carried out as a multi-stage process that includes four process stages: (1) the preparation of the catalyst (precarbonylation), (2) the catalyst extraction, (3) the olefin hydroformylation, and (4) the removal of the catalyst from the reaction product (decobalting). Since the development of the oxo synthesis, the individual process stages of the cobalt-catalyzed hydroformylation are continually being improved and modified.
In the first process stage, the precarbonylation, the catalyst complex (HCo(CO).sub.4) required for the hydroformylation is prepared starting from an aqueous cobalt salt solution by reaction with carbon monoxide and hydrogen. According to DE-OS 2,139,630, the precarbonylation is preferably carried out at temperatures from 100 to 160.degree. C. and under synthesis gas pressures from 200 to 300 bar in the presence of activated carbon, zeolites or basic ion exchangers loaded with cobalt carbonyls.
DE-OS 2,244,373 describes an improved continuous carbonylation process, in which marked shortening of the reaction time is achieved by concurrently passing the starting materials, synthesis gas and aqueous cobalt salt solution, in the presence of oxygen-containing organic solvents, which are poorly miscible with water or not at all, through a zone in which a turbulent flow is maintained. As an advantageous embodiment, the use of a pressurized turbulence pipe for maintaining the turbulent flow and the addition of alcohols or aldehydes having 4 to 10 carbon atoms as the organic solvent are mentioned.
In the second process stage, the catalyst extraction, the cobalt catalyst prepared in the first process stage is extracted from the aqueous phase with an organic phase, preferably the olefin which is to be hydroformylated. According to DE-OS 2,106,252, it is expedient to employ for the catalyst extraction, in addition to the olefin, the reaction products and by products of the hydroformylation, provided they are water-insoluble and liquid under the selected reaction conditions. The catalyst extraction is preferably carried out in a countercurrent at temperatures from 20 to 100.degree. C. and under synthesis gas pressures from 100 to 400 bar. After the phase separation, the organic phase loaded with the cobalt catalyst is fed to the third process stage, the hydroformylation.
From DE-OS 2,139,630, it is known that, in the third process stage, the hydroformylation, olefins loaded with the cobalt catalyst can be hydroformylated in a high-pressure reactor with synthesis gas at temperatures between 70 and 170.degree. C. and at pressures from 100 to 400 bar to give the corresponding aldehydes. Some of the aldehydes formed can be hydrogenated to the alcohol under the hydroformylation conditions, particularly at high temperatures.
The reaction product that, in addition to the valuable aldehyde and alcohol, contains by-products, residual olefin which is not hydroformylated and the cobalt catalyst, is let down to 1 to 15 bar and then fed to the catalyst reprocessing stage.
In the fourth process stage, the decobalting, the organic phase of the reaction product is freed of the cobalt carbonyl complexes in the presence of complex-free process water by treatment with oxygen or air. According to WO 93/24438, the decobalting is carried out at temperatures from 60 to 100.degree. C. and pressures from 1 to 20 bar. In this way, the cobalt catalyst is oxidatively destroyed and the resulting cobalt salts are back-extracted into the aqueous phase. The resulting aqueous cobalt salt solution from the decobalting is recycled into the first process stage, the precarbonylation.
A further embodiment is described in WO 93/24437 and EP-OS 0,183,546. In this case, gas scrubbing with synthesis gas or nitrogen is performed before the oxidative destruction of the cobalt catalyst.
After the gas phase has been separated off, the reaction products in the remaining organic phase are converted to the corresponding alcohols in further processing stages, such as hydrogenation and distillation.
The known multi-stage production processes for oxo aldehydes in the presence of cobalt catalysts have a number of industrial disadvantages. Two expensive process stages, precarbonylation and catalyst extraction, are required for preparing the cobalt catalyst needed for hydroformylation. Due to the mass transfer processes occurring in the two process stages, namely gas/liquid mass transfer in the precarbonylation and liquid/liquid mass transfer in the catalyst extraction, two separate pressure-resistant modules, for example, stirred vessels or packed columns, are necessary. The actual hydroformylation subsequently takes place in a separate pressure reactor. The removal of the cobalt catalyst is tied to a further plant section.
Therefore, the known multi-stage hydroformylation processes necessarily require a very high investment, in addition to a large process engineering effort.
The present invention is therefore based on the object of developing a novel hydroformylation process for olefins and subsequent hydrogenation of the resulting aldehydes that is more economical and easier to carry out in process engineering terms.
It has now been found, surprisingly, that the formation of the cobalt catalyst, the extraction of the cobalt catalyst formed into the organic phase and the hydroformylation of the corresponding olefins can be carried out in a 1-stage process.